Furnaces don't work like that. Everything on the 24 volt circuit depends on flame rectification. meaning if there was ever gas flow and no flame sensed, the circuit would go back to its normally open position on the gas valve, killing the gas altogether.

The cat was being boarded? I have cats, you can leave them home for a few days with a bowl of food and a bowl of water. The worst you'll get is some crap on the floor when they get pissed that the box hasn't been scooped. Who boards their cat for an evening out? That's like $30 a day, for a family who's heat went out a week and a half before that and presumably hadn't been fixed yet. In November (ie they're broke).

Manslaughter, maybe. But a gas leak? Sure it could...we had one in Boston a couple years back, did about as much damage.

Paramedic I used to know told me gas explosions were weird calls...you'd have two dead...REALLY dead, no issues there...and about two dozen cases of cut feet, from all the neighbors getting up in the middle of the night to find out what blew their windows in...